A tap drill size chart tells you which drill bit to use before threading a hole with a tap. The tap drill is slightly smaller than the thread's outer diameter, leaving material for the tap to cut. This reference covers UNC (coarse), UNF (fine), and metric threads with drill bit designations, decimal sizes, and a thread percentage calculator.
Tap Drill Size Chart
| Thread Size | TPI / Pitch | Tap Drill | Decimal (in) | Drill (mm) |
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Thread Percentage Calculator
Calculate the required tap drill diameter for any thread percentage (standard is 75%). Works for both inch and metric threads.
Clearance Hole Sizes
Clearance holes allow a bolt to pass through freely without threading. Three classes: Close (±0.002"), Normal, and Loose.
| Thread Size | Close | Normal | Loose |
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How to Use the Tap Drill Size Chart
This free tap drill size chart covers all three major thread standards — UNC (Unified National Coarse), UNF (Unified National Fine), and metric (ISO) — so you can quickly find the correct drill bit before tapping any threaded hole. Whether you're machining steel, drilling aluminum, or working with plastic, the right tap drill size prevents broken taps and stripped threads.
Step 1: Select the Thread Standard
Use the tabs at the top to filter the chart to UNC, UNF, or Metric threads. If you're unsure, use the search box to type the thread designation — for example, 1/4-20 for inch threads or M6 for metric. The table filters instantly as you type.
Step 2: Read the Tap Drill Column
The Tap Drill column shows the drill bit designation to use for 75% thread engagement — the industry standard for most applications. For inch threads, this is a wire gauge number (like #7), a letter (F), or a fractional size (5/16"). For metric threads, it is always a millimeter size (5.0mm).
Step 3: Use the Thread Percentage Calculator for Non-Standard Engagement
If you need a different thread engagement percentage — for example, 50% for soft plastics or 85% for high-strength steel joints — use the Thread Percentage Calculator. Enter the nominal diameter, TPI or pitch, and desired percentage. The tool calculates the required drill diameter in both inches and millimeters. For inch threads: drill diameter = nominal diameter − (percentage ÷ 76.98) × (1 ÷ TPI). For metric threads the pitch in mm is used directly.
Understanding UNC vs. UNF Threads
UNC (Unified National Coarse) threads are the default for most fasteners in the US. They have a lower TPI (fewer threads per inch), assemble faster, and tolerate slight misalignment. UNF (Unified National Fine) threads have higher TPI, providing stronger clamping force and better resistance to loosening from vibration. A 1/4" bolt, for example, is 1/4"-20 in UNC and 1/4"-28 in UNF — the UNF version needs a slightly larger tap drill (letter I vs. #7) because the finer pitch means shallower thread depth.
Clearance Holes vs. Tap Drill Holes
A tap drill hole is used when you want to cut threads and have the fastener thread into the material. A clearance hole is larger — it allows the bolt shank to pass through freely with no thread contact. Clearance holes come in three classes: Close (tight fit, ±0.002"), Normal (general use), and Loose (easy alignment). Use the clearance hole reference table at the bottom of this page when you need a through-hole rather than a tapped hole.
Tap Drill Tips for Success
Always use cutting fluid when tapping steel or stainless steel — it reduces tap wear and breakage significantly. For blind holes (holes that don't go all the way through), drill slightly deeper than the thread depth needed to allow chip clearance. Use a spiral-flute tap in blind holes to pull chips up and out. For through-holes, a straight-flute tap works fine. Back off the tap a quarter turn every two turns to break chips. Never force a tap — if resistance increases suddenly, back out and clear chips before continuing.
FAQ
Is this tap drill size chart free to use?
Yes, completely free with no account or signup required. All data runs locally in your browser — nothing is sent to any server. Bookmark it for quick shop access anytime.
Is my data private when using this tool?
Yes, all calculations happen in your browser. No data is transmitted to any server. The tool works entirely offline once the page has loaded.
What is a tap drill and why does it matter?
A tap drill is the drill bit you use to create a hole before threading it with a tap. The tap drill size is slightly smaller than the thread's nominal diameter, leaving enough material for the tap to cut clean thread form. Choosing the wrong size either strips the thread or causes the tap to break.
What is 75% thread engagement and why is it the standard?
Thread engagement percentage refers to how much of the theoretical full thread depth is actually cut. 75% thread engagement provides about 90–95% of the strength of a full 100% thread, while being significantly easier to tap and causing much less tap breakage. For most applications in steel, aluminum, and plastic, 75% is the industry-standard recommendation.
What is the difference between UNC and UNF threads?
UNC (Unified National Coarse) threads have fewer threads per inch (TPI), making them faster to assemble and more forgiving in softer materials. UNF (Unified National Fine) threads have more TPI, providing greater clamping force and better vibration resistance. UNF threads require a slightly larger tap drill since the thread depth is shallower relative to the bolt diameter.
How do I calculate the tap drill size for any thread percentage?
Use the thread percentage calculator on this page. Enter the thread nominal diameter and pitch (or TPI), choose your desired thread engagement percentage (50–100%), and the calculator returns the required drill diameter in both inches and mm. The formula is: Drill diameter = Nominal diameter − (Thread percentage / 76.98 × Pitch).
What tap drill do I use for M6 x 1.0 metric threads?
For an M6×1.0 metric thread at 75% engagement, use a 5.0mm (0.197") drill bit. This is one of the most common metric thread sizes and the 5mm drill is standard in nearly every drill set.
What is the tap drill for 1/4-20 UNC?
For a 1/4"-20 UNC thread, the standard tap drill is a #7 drill bit, which measures 0.201" (5.11mm). This gives approximately 75% thread engagement in steel or aluminum.